enable maven plugin to run multiple times in multiple executions

Currently the exam-maven-plugin doesn't support multiple executions because of calling setURLStreamHandlerFactory. Also the plugin context map is not suitable for that. The pull request contains two fixes:
* quickfix for errors resulting of calling setURLStreamHandlerFactory multiple times. There is probably a better way than catching the error.
* Extend the context map key with the execution id so that different pax instances can be safely shut down at the end.